1. Field of the Invention
This invention relates generally to modulating fatty acid profiles of soybean seed through genetic engineering and the resulting soybean oil compositions. Recombinant DNA constructs, soybean plants and seeds, and soybean oil composition with altered fatty acid profile are provided.
2. Related Art
Plant oils are used in a variety of applications. Novel vegetable oil compositions and improved approaches to obtain oil compositions, from synthetic or natural plant sources, are needed. Depending upon the intended oil use, various fatty acid compositions are desired. Plants, especially species which synthesize large amounts of oils in seeds, are an important source of oils both for edible and industrial uses. Seed oils are composed almost entirely of triacylglycerols in which fatty acids are esterified to the three hydroxyl groups of glycerol.
Soybean oil typically contains about 11-17% saturated fatty acids: 8-13% palmitate and 3-4% stearate. See generally Gunstone et al., The Lipid Handbook, Chapman & Hall, London (1994). Soybean oil has been modified by various breeding methods to create benefits for specific markets. However, for the production of most baked goods and coatings there is a need for high solids-containing stable fats. In the past, partially hydrogenated soybean oil was used for this purpose, but with the introduction of trans fatty acid labeling, the desirability of this oil has decreased.
Higher plants synthesize fatty acids via a common metabolic pathway—the fatty acid synthetase (FAS) pathway, which is located in the plastids. β-ketoacyl-ACP synthases are important rate-limiting enzymes in the FAS of plant cells and exist in several versions. β-ketoacyl-ACP synthase I catalyzes chain elongation to palmitoyl-ACP (C16:0), whereas β-ketoacyl-ACP synthase II catalyzes chain elongation to stearoyl-ACP (C18:0). In soybean, the major products of FAS are 16:0-ACP and 18:0-ACP. The desaturation of 18:0-ACP to form 18:1-ACP is catalyzed by a plastid-localized soluble delta-9 desaturase (also referred to as “stearoyl-ACP desaturase”). See Voelker et al., 52 Annu. Rev. Plant Physiol. Plant Mol. Biol. 335-61 (2001).
The products of the plastidial FAS and delta-9 desaturase, 16:0-ACP, 18:0-ACP, and 18:1-ACP, are hydrolyzed by specific thioesterases (FAT). Plant thioesterases can be classified into two gene families based on sequence homology and substrate preference. The first family, FATA, includes long chain acyl-ACP thioesterases having activity primarily on 18:1-ACP. Enzymes of the second family, FATB, commonly utilize 16:0-ACP (palmitoyl-ACP), 18:0-ACP (stearoyl-ACP), and 18:1-ACP (oleoyl-ACP). Such thioesterases have an important role in determining chain length during de novo fatty acid biosynthesis in plants, and thus these enzymes are useful in the provision of various modifications of fatty acyl compositions, particularly with respect to the relative proportions of various fatty acyl groups that are present in seed storage oils.
The products of the FATA and FATB reactions, the free fatty acids, leave the plastids and are converted to their respective acyl-CoA esters. Acyl-CoAs are substrates for the lipid-biosynthesis pathway (Kennedy Pathway), which is located in the endoplasmic reticulum (ER). This pathway is responsible for membrane lipid formation as well as the biosynthesis of triacylglycerols, which constitute the seed oil. In the ER there are additional membrane-bound desaturases, which can further desaturate 18:1 to polyunsaturated fatty acids. A delta-12 desaturase (FAD2) catalyzes the insertion of a double bond into oleic acid (OA) (18:1), forming linoleic acid (LA) (18:2). A delta-15 desaturase (FAD3) catalyzes the insertion of a double bond into 18:2, forming alpha linolenic acid (ALA) (18:3).
Inhibition of the endogenous FAD2 genes through use of transgenes that silence the expression of FAD2 has been shown to confer a desirable oleic acid (18:1) phenotype (i.e. soybean seed comprising about 50% and 75% oleic acid by weight). Transgenes and transgenic plants that provide for inhibition of the endogenous FAD2 gene expression and a desirable oleic phenotype are disclosed in U.S. Pat. No. 7,067,722. In contrast, soybean cultivars that lack FAD2-inhibiting transgenes typically produce seed with oleic acid compositions of less than 20%.
Soybean oil typically contains about 8% ALA (18:3) that renders this oil oxidatively unstable. The levels of ALA in soybean oil can be reduced by hydrogenation to improve both stability and flavor. Unfortunately, hydrogenation results in the production of trans-fatty acids, which increases the risk for coronary heart disease when consumed.
Oleic acid has one double bond, but is still relatively stable at high temperatures, and oils with high levels of OA are suitable for cooking and other processes where heating is required. Recently, increased consumption of high OA oils has been recommended, because OA appears to lower blood levels of low density lipoproteins (“LDLs”) without affecting levels of high density lipoproteins (“HDLs”). However, some limitation of OA levels is desirable, because when OA is degraded at high temperatures, it creates negative flavor compounds and diminishes the positive flavors created by the oxidation of LA. Neff et al., JAOCS, 77:1303-1313 (2000); Warner et al., J. Agric. Food Chem. 49:899-905 (2001). It is thus preferable to use oils with OA levels that are 65-85% or less by weight, in order to limit off-flavors in food applications such as frying oil and fried food. Other preferred oils have OA levels that are greater than 55% by weight in order to improve oxidative stability.